The present invention relates to a process for the precision molding of glass articles such as glass lenses. More particularly, the invention relates to improved mold materials permitting the direct molding of precision glass lenses from selected directly moldable optical glass compositions at economical rates without undue mold wear or other deterioration of the surface quality of the molds.
Precision optical elements such as optical lenses generally require refracting surfaces of precise configuration and a high degree of surface smoothness. Moreover, opposing surfaces of the optical elements must be precisely aligned in order to provide the optical focusing characteristics required for a particular end use.
Precision optical elements of glass are presently made using a multistep process wherein a molten glass batch for a glass having the required refractive index and other characteristics is first melted to provide molten optical glass. A suitable charge of such glass is then formed by pressing or other conventional process to yield a lens shape approximating the desired final shape for the product.
Conventional pressing processes do not readily provide a lens having a surface figure and surface finish suitable for direct optical use. Rather, the lens must first be annealed to relieve internal stresses and insure refractive index homogeneity, and then conventionally ground and polished to achieve the final lens configuration and surface smoothness.
Conventional grinding and polishing processes are subject to fundamental limitations as to the surface profiles which may be obtained. The surface profiles that can be produced by conventional machine grinding and polishing techniques are normally restricted to conic sections, such as flats, spheres, and parabolas. Other shapes and, in particular, general aspheric surface configurations, are difficult to form by such grinding. In addition, it is difficult to polish a ground surface having a pre-established surface figure to obtain the necessary optical finish without altering the pre-established surface configuration and thus changing the optics of the lens.
For these reasons, the production of glass aspheric lenses by conventional techniques requires highly skilled and expensive hand working. Critical aspects of this procedure include obtaining the necessary surface finish by grinding and/or polishing without causing some misalignment of opposing aspheric surfaces of the lens, and holding surface figure (shape) through any necessary surface finishing. Either of misalignment or loss of figure renders the finished lens useless for the intended application.
These difficulties have been recognized and the art is well aware that direct molding of aspheric lenses to a surface-finished state could theoretically eliminate the grinding, polishing, and edging which so greatly add to the cost of aspheric lenses. This approach has already proven practicable for plastic lenses. However existing plastics suitable for optical applications are available only in a limited refractive index and dispersion range. Furthermore plastics are subject to damage via scratching, yellowing, and haze. And, the use of compatible abrasion-resistant and/or anti-reflective coatings on plastic lenses has not fully met the need for a durable, stable lens material.
Plastic lenses are also prone to distortion from mechanical force, humidity, and heat. Hence both the volume and refractive index of common plastics can vary substantially with changes in temperature, thereby limiting the useful temperature range within which the lens will function according to optical design criteria.
Because of these shortcomings, repeated attempts to provide finished optical surfaces by the direct pressing of hot glass have been made. However, several fundamental difficulties must be overcome before the direct pressing of glass lenses can be achieved. For example, conventional glass pressing processes typically produce chill wrinkles in the pressed surface in the product and/or surface irregularities deviating from the figure of the molding surface.
A number of techniques have been developed to correct the shortcomings of conventional hot glass pressing processes. Among these are special pressing apparatus utilizing isothermal pressing, i.e., pressing using heated molds and preheated glass so that the temperatures under which the pressing step is carried out vary only slightly across the glass preform during the pressing interval. Special materials to construct the molds and special glass compositions and molding process parameters have also been developed in attempts to improve the quality of direct-pressed lenses.
U.S. Pat. No. 3,244,497 describes a lens blank molding apparatus wherein a temperature-controlled plunger and an insulated mold base offering controllable heat transfer to a supporting press table are described. However the apparatus is designed for the pressing of relatively thin lens blanks, this factor being an important contributor to the temperature control attainable with the apparatus. U.S. Pat. No. 4,481,023 describes alternative molding apparatus for the direct pressing of lenses of optical quality. Again, temperature control of the molding surfaces is provided, and the apparatus is designed for pressing at rather high glass viscosities of 10.sup.8 -10.sup.12 poises. This corresponds to a relatively low pressing temperature, which helps to reduce difficulties stemming from non-uniform heat flow.
The use of mold coatings to enhance the surface quality of the pressings, to improve mold durability, and to act as a parting agent from the molten glass is suggested in U.S. Pat. No. 3,244,497, supra. Refractory coatings selected from the group consisting of refractory nitrides, borides, carbides, and oxides are suggested. Coatings no thicker than approximately half the wavelength of visible light, e.g. 0.5 microns, are suggested in order that the coating faithfully reproduce the mirror finish of the underlying mold surface.
U.S. Pat. No. 4,168,961 describes a method for the precision molding of optical glass elements wherein a mold having molding surfaces of a silicon carbide/glassy carbon mixture is proposed. The patent suggests that glass elements molded against this material exhibit high surface quality and surface accuracy. However molding under a controlled atmosphere is required to avoid oxidation of this material, a circumstance which substantially reduces the practical economic value of the method.
U.S. Pat. No. 4,139,677 proposes the precision molding of optical glass elements in a mold having molding surfaces formed of silicon carbide or silicon nitride. This method also reportedly provides good surface quality and configuration but, again, an oxygen-free atmosphere within the molding chamber must be maintained to avoid oxidation of the mold coatings.
In order to realize the economical advantages of direct molding for products such as aspheric lenses, factors relating to the service life of the molds employed for the pressing operation must be taken into account. The machining of aspheric shapes in molds makes them relatively expensive, particularly since very hard and durable mold materials are generally required. This is especially true for molding processes involving low-temperature, high viscosity molding, because of the higher molding stresses involved.
Primary factors affecting mold life include chemical reactions occurring between the hot mold and the molten glass, and between the hot mold and the atmosphere. The latter factor is particularly significant when rapid production rates prohibiting cooling of the shaped lens in the mold are desired. And, as previously noted, while prior art approaches have suggested the use of a controlled atmosphere for molding to avoid oxidation or other degradation of the mold surface, such a limitation is inconsistent with rapid and economical lens production.
The development of direct lens molding in glass has been substantially aided by the discovery of new glass compositions which can be molded at relatively low pressing temperatures, yet which are not subject to attack by moisture in the manner usual for soft glasses. U.S. Pat. No. 4,362,819 discloses examples of alkali aluminofluorophosphate glasses useful for such applications. However, the pressing of such glasses at economical rates has been difficult because of limited compatibility between these glasses and conventional mold materials.
It is therefore a principal object of the present invention to provide a mold construction and molding process which can be practiced in a normal air atmosphere yet which will provide long mold lifetime due to the high wear resistance and high chemical resistance of the mold surface to hot glass.
It is a further object to provide a mold construction which can be used in a low-temperature, high viscosity, high pressure process.
It is a further object of the invention to provide a mold construction and molding process useful to make aspheric glass molding elements closely matching a selected aspheric surface figure and providing pressed glass elements of accurate surface figure and good surface finish.
It is a further object of the invention to provide a mold construction and a molding process which can be used to directly form optical surface figures and optical finishes on lens elements formed of moldable alkali aluminofluorophosphate glass compositions or the like which have shown particularly advantageous properties for the production of molded optical elements.
Other objects and advantages of the invention will become apparent from the following description thereof.